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Transcript
SQL is a standard computer language for accessing and manipulating databases. What is SQL? SQL stands for Structured Query Language SQL allows you to access a database SQL is an ANSI standard computer language SQL can execute queries against a database SQL can retrieve data from a database SQL can insert new records in a database SQL can delete records from a database SQL can update records in a database SQL is easy to learn SQL is a Standard - BUT.... SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database. SQL works with database programs like MS Access, DB2, Informix, MS SQL Server, Oracle, Sybase, etc. Unfortunately, there are many different versions of the SQL language, but to be in compliance with the ANSI standard, they must support the same major keywords in a similar manner (such as SELECT, UPDATE, DELETE, INSERT, WHERE, and others). Note: Most of the SQL database programs also have their own proprietary extensions in addition to the SQL standard! SQL Database Tables A database most often contains one or more tables. Each table is identified by a name (e.g. "Customers" or "Orders"). Tables contain records (rows) with data. Below is an example of a table called "Persons": LastName Hansen Svendson FirstName Ola Tove Address Timoteivn 10 Borgvn 23 City Sandnes Sandnes Pettersen Kari Storgt 20 Stavanger The table above contains three records (one for each person) and four columns (LastName, FirstName, Address, and City). SQL Queries With SQL, we can query a database and have a result set returned. A query like this: SELECT LastName FROM Persons Gives a result set like this: LastName Hansen Svendson Pettersen Note: Some database systems require a semicolon at the end of the SQL statement. We don't use the semicolon in our tutorials. SQL Data Manipulation Language (DML) SQL (Structured Query Language) is a syntax for executing queries. But the SQL language also includes a syntax to update, insert, and delete records. These query and update commands together form the Data Manipulation Language (DML) part of SQL: SELECT - extracts data from a database table UPDATE - updates data in a database table DELETE - deletes data from a database table INSERT INTO - inserts new data into a database table SQL Data Definition Language (DDL) The Data Definition Language (DDL) part of SQL permits database tables to be created or deleted. We can also define indexes (keys), specify links between tables, and impose constraints between database tables. The most important DDL statements in SQL are: CREATE TABLE - creates a new database table ALTER TABLE - alters (changes) a database table DROP TABLE - deletes a database table CREATE INDEX - creates an index (search key) DROP INDEX - deletes an index The SELECT Statement The SELECT statement is used to select data from a table. The tabular result is stored in a result table (called the result-set). Syntax SELECT column_name(s) FROM table_name Select Some Columns To select the columns named "LastName" and "FirstName", use a SELECT statement like this: SELECT LastName,FirstName FROM Persons "Persons" table LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Result LastName Hansen FirstName Ola Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger Svendson Pettersen Tove Kari Select All Columns To select all columns from the "Persons" table, use a * symbol instead of column names, like this: SELECT * FROM Persons Result LastName Hansen Svendson Pettersen FirstName Ola Tove Kari Address Timoteivn 10 Borgvn 23 Storgt 20 City Sandnes Sandnes Stavanger The Result Set The result from a SQL query is stored in a result-set. Most database software systems allow navigation of the result set with programming functions, like: Move-To-FirstRecord, Get-Record-Content, Move-To-Next-Record, etc. Programming functions like these are not a part of this tutorial. To learn about accessing data with function calls, please visit our ADO tutorial. Semicolon after SQL Statements? Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server. Some SQL tutorials end each SQL statement with a semicolon. Is this necessary? We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it. The SELECT DISTINCT Statement The DISTINCT keyword is used to return only distinct (different) values. The SELECT statement returns information from table columns. But what if we only want to select distinct elements? With SQL, all we need to do is to add a DISTINCT keyword to the SELECT statement: Syntax SELECT DISTINCT column_name(s) FROM table_name Using the DISTINCT keyword To select ALL values from the column named "Company" we use a SELECT statement like this: SELECT Company FROM Orders "Orders" table Company Sega W3Schools Trio W3Schools OrderNumber 3412 2312 4678 6798 Result Company Sega W3Schools Trio W3Schools Note that "W3Schools" is listed twice in the result-set. To select only DIFFERENT values from the column named "Company" we use a SELECT DISTINCT statement like this: SELECT DISTINCT Company FROM Orders Result: Company Sega W3Schools Trio Now "W3Schools" is listed only once in the result-set. The WHERE clause is used to specify a selection criterion. The WHERE Clause To conditionally select data from a table, a WHERE clause can be added to the SELECT statement. Syntax SELECT column FROM table WHERE column operator value With the WHERE clause, the following operators can be used: Operator = <> > < >= <= BETWEEN LIKE Description Equal Not equal Greater than Less than Greater than or equal Less than or equal Between an inclusive range Search for a pattern Note: In some versions of SQL the <> operator may be written as != Using the WHERE Clause To select only the persons living in the city "Sandnes", we add a WHERE clause to the SELECT statement: SELECT * FROM Persons WHERE City='Sandnes' "Persons" table LastName Hansen Svendson Svendson Pettersen FirstName Ola Tove Stale Kari Address Timoteivn 10 Borgvn 23 Kaivn 18 Storgt 20 City Sandnes Sandnes Sandnes Stavanger Year 1951 1978 1980 1960 City Sandnes Sandnes Sandnes Year 1951 1978 1980 Result LastName Hansen Svendson Svendson FirstName Ola Tove Stale Address Timoteivn 10 Borgvn 23 Kaivn 18 Using Quotes Note that we have used single quotes around the conditional values in the examples. SQL uses single quotes around text values (most database systems will also accept double quotes). Numeric values should not be enclosed in quotes. For text values: This is correct: SELECT * FROM Persons WHERE FirstName='Tove' This is wrong: SELECT * FROM Persons WHERE FirstName=Tove For numeric values: This is correct: SELECT * FROM Persons WHERE Year>1965 This is wrong: SELECT * FROM Persons WHERE Year>'1965' The LIKE Condition The LIKE condition is used to specify a search for a pattern in a column. Syntax SELECT column FROM table WHERE column LIKE pattern A "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern. Using LIKE The following SQL statement will return persons with first names that start with an 'O': SELECT * FROM Persons WHERE FirstName LIKE 'O%' The following SQL statement will return persons with first names that end with an 'a': SELECT * FROM Persons WHERE FirstName LIKE '%a' The following SQL statement will return persons with first names that contain the pattern 'la': SELECT * FROM Persons WHERE FirstName LIKE '%la%'